1. Field of the Invention
Embodiments of the present invention relate, in general, to flip chip and wirebond packaging structures and methods for manufacturing the same and more particularly to flip chip and wirebond on flexible substrates with matching coefficient of thermal expansion stiffeners and their corresponding manufacturing methods.
2. Relevant Background
In semiconductor devices assembly, a semiconductor chip (also referred to as an integrated circuit (IC) chip or “die”) may be bonded directly to a packaging substrate. Recall that traditional semiconductor fabrication chips are built in large numbers on a single wafer wherein individual chips are patterned to have small pads of metal that serve as connection points to an eventual mechanical carrier or substrate. The chip is then cut out of the wafer and attached to the substrate typically by a wire bonding process. These traces lead to pins which are attached to the rest of the circuitry making up an electronic system.
Flip chip processing is similar to conventional die fabrication with a few additional steps. At the end of the manufacturing process attachment pads are metalized to make them more receptive to flip chip bumps. A small amount of conductive material such as gold or solder (referred to as a bump) is deposited on each of the metal pads. The chips are then cut out of the wafer and flipped or inverted to bring the bumps down onto pads of the underlying substrate, electronics or circuit board. The bumps are then connected via various assembly processes to produce an electrical connection. Packages of this type are referred to as flip chip packages.
In a conventional flip chip package, the die and packaging substrate are electrically connected and mechanically bonded in a solder joining operation. The die is aligned with and placed onto a packaging substrate such that the die's solder bumps are aligned with the metal pads on the substrate. Heat is applied causing the solder bumps to melt and alloy with the substrate pads forming electrical/mechanical connections between the die and the packaging substrate.
Underfill is then applied in order to enhance the mechanical bonding of the die and substrate. Underfill material is typically a thermal-set epoxy dispensed into the remaining space or gap between the die and substrate. In order to improve thermal performance and reliability of the package, heat spreaders are often used. A heat spreader may be of one-piece or multiple-piece construction. Occasionally, a stiffener is placed around the die on the same side of the substrate. The stiffener is typically a flat piece of high modulus metal having substantially same dimensions as the package substrate with a window in the center to clear the die. The purpose of the stiffener is to constrain the substrate to prevent warping or other movement which may be caused by thermal cycling during reliability testing or field operations.
Flip chip processing is not without its disadvantages. The chips require a very flat surface during bonding and it is difficult to arrange and maintain alignment as the boards heat and cool. Furthermore, the connections between the die and substrate are very short and stiff limiting their ability to withstand thermal expansion.
Another problem with flip chip package construction is that during the cool down from the solder joining temperatures and underfill curing, the entire package is highly stressed due to different coefficient for thermal expansion (CTE) of the substrate and die material. This problem is exasperated in the case of relatively large dies.
The semiconductor industry continually seeks to improve the electrical and mechanical performance of semiconductor die assemblies. However, thinner package substrates are more susceptible to warping resulting from CTE mismatches of material within the package as well as less inherent strength of the thin materials themselves.
Another advance in semiconductor packaging is the utilization of a flexible substrate. The top layer is a double-sided, flexible laminate to which chips are often flip chip attached. That is, solder joints are used to connect the die to the flexible substrate. The flexible nature of the substrate exasperate the challenges presented in assembly. Thermal management, substrate flatness and alignment during manufacturing all become major issues. These and other challenges of the prior art are addressed by one or more embodiments of the present invention, hereafter described by way of example.